The present invention relates to earth drains adapted to the drainage of soil having low water permeability such, for example, as clay.
Certain soils such as clay have low water permeability and do not drain well with the result that such soils are often inappropriate for the purpose for which the land is intended, be it the construction of residential or commercial buildings, the construction of road beds or highways, or for agricultural or similar purposes. Such soils can be rendered more suitable for the purpose for which they are intended by the provision of drains which permit the pore water to be drained away before any intended construction is carried out. A basic drain used for this purpose is a conventional sand drain which is formed by inserting pipes into the ground through the layer of soil having low water permeability and into a layer of soil having relatively high water permeability such as sand or silt which drains water more readily. The pipe is filled with sand and then withdrawn leaving a column of sand extending through the soil layer having low permeability into the soil layer having relatively high permeability. The resulting sand column permits ground water to pass readily through the clay layer into the layer of relatively high permeability where it may be dispersed by normal drainage; alternately, the sand column may permit the ground water to escape upward to the ground surface. Such sand drain also permits drainage of pore water in the soil having low permeability, which pore water is squeezed out of the ground by a load, for example, a fill surcharge on the ground surface. The pore water is squeezed into the same column, which offers less resistance to the flow of water than the soil of low permeability, and which then permits the pore water to drain into the soil exhibiting relatively high water permeability, or to escape to the ground surface.
A drain which forms a substitute for the aforementioned sand column, and which may also be suitable for irrigation or similar purposes, is illustrated in Wager Canadian Pat. No. 930,999, dated July 31st, 1973. This drain is the type of drain to which this invention relates and consists of a relatively flat core consisting of a wave-shaped strip of plastic which is encased in a filter of water permeable sheet-like material. The troughs in the wave-shaped strip form channels for the dispersion of water and the filter material permits the passage of water therethrough into the troughs but prevents the ingress of soil or other particulate matter which would tend to block the channels formed by the troughs and thus deleteriously affect the performance of the drain. While this type of drain has many advantages over the basic sand drain, it also has certain disadvantages arising in part from the fact that there is no communication between the water carrying channels formed by the troughs in the wave-shaped strip of the core. Thus if one of the channels becomes blocked at any point throughout its length, such as by tearing or breakage of the filter material to permit the ingress of sand or particulate material, or by the soil pressure forcing the filter material into the troughs of the wave-shaped strip, the particular channel or channels so blocked become useless insofar as the drainage of water through the point of blockage is concerned. If many or all channels become blocked, the drain becomes ineffective. Further, even if none of the water carrying channels are blocked or their flow areas reduced by the earth pressure forcing the filter material into the channel, the effective flow area of such drains is relatively small owing to the relatively low ratio of open area to solid area.